1. Field of the Invention
The invention relates to a method of echo cancellation for a full duplex loudspeaking telephone, in which the loudspeaker signal picked up by the microphone is cancelled by means of an adaptive filter. It also relates to a device for echo cancellation with an adaptive filter.
2. Description of the Related Art
Such a method and a corresponding device are known, for example from EP 0,301,627 B1. Said document describes an adaptive filter which operates substantially free from delay. The coefficients of the adaptive filter can be adjusted only in those time intervals in which there is no speech at this location, so that the microphone then only picks up the echo signal, which is cancelled to an optimum extent by means of the adaptive filter.
For a correct operation of the adaptive filter it is essential that the levels of the signals at the input of the adaptive filter and the combining circuit, i.e. the summing circuit, are in a given ratio to one another. This is generally achieved by arranging an amplifier before the loudspeaker and an amplifier after the microphone, the gain of the two amplifiers being adjusted in such a manner that the ratio between the levels of the two signals has a given value. However, these levels depend on the geometrical position of the loudspeaker relative to the microphone and on the environment, and the adjustment must be repeated upon every change of place.